1. Technical Field
This invention relates to communication systems and receivers and more particularly to wide area paging systems and portable paging receivers with low power consumption and method of data transmission to many receivers on a common channel.
2. Description of Prior Art
Prior paging systems generally provide two categories of coverage. Local area coverage is typically provided by transmitting a page simultaneously from one or more transmitters operating on a single frequency, covering all or a portion of a metropolitan area, to a recipient within the local area. Such coverage is obviously limited to the broadcast range of a local transmitter. Wide area coverage, encompassing several overlapping areas of a large metropolitan area or a number of non-overlapping metropolitan areas, is typically provided by transmitting a paging request to several transmitters via telecommunications and broadcasting the paging message from all transmitters to an intended recipient who may be anywhere within the wide coverage area. The foregoing approach to providing a wide area coverage is expensive and limited to a relatively small number of paging users. As paging traffic increases, the local and wide area coverage users begin to complete for paging channel space. Each wide area user occupies a portion of the available paging channel space for all of the page transmitters within the wide coverage area, displacing local coverage users of the same channel portion from each transmitter. It would be preferable to be able to provide wide area paging coverage without displacing local area coverage users of the same channel portion from all transmitters in the wide area.
Another problem with present paging systems is their incompatability. Local area coverage users may use one type of paging message encoding scheme for broadcasting a page to a page receiver and wide area coverage users may use a second type. A user who travels frequency must carry different page receivers to receive both types of messages. One solution to the incompatibility is disclosed in U.S. Pat. No. 4,518,961 to Davis et al. Davis et al. describes a single page receiver that stores the encoding schemes for a number of paging systems, encoding schemes such as POCSAG, the paging protcol for the British Post Office, and GGC, a paging protocol designed by Motorola, Inc. This is only a partial solution, however, because the number of encoding schemes greatly exceeds the number the Davis et al. receiver can store.
Prior paging systems have also used various methods for reducing the power consumption of battery-operated portable page receivers to minimize their overall size and to extend their battery life. One approach involves supplying power to receiver circuitry in the page receiver only during a predetermined time slot in which data for the particular page receiver is transmitted. The drawbacks of this approach, however, have been several. First, it is difficult to synchronize accurately the page receiver to the transmitter. Second, long messages that cannot be transmitted in a single time slot require an inordinate amount of time to be completely transmitted to a page receiver. Third, page receivers using this approach have largely relied on a single communications channel, which may not be strong enough in a particular area to transmit successfully to the page receiver. Finally, it has so far proven impractical to implement pagers with miniature batteries because previous pagers do not have low enough power consumption to avoid frequent battery replacement or recharging.
One such prior art paging system is disclosed in U.S. Pat. No. 3,937,004 to Natori et al. The Natori et al. patent describes a page receiver in the form of wristwatch that activates its receiver circuitry periodically for a given time interval to detect a paging signal that may be directed to it. In the particular embodiment given, the receiver is activated five minutes out of a fifteen minute transmission cycle. Although this technique does reduce power consumption, it still requires receiving circuitry to be on one-third of the time.
A battery-saving arrangement for pagers is also disclosed in U.S. Pat. No. 4,398,192 to Moore et al. Page receivers are assigned to groups, and the receivers of each group are activated during a time segment of a transmission cycle set aside for the group. Each receiver within a particular group then is active during the entire group time segment to detect any individually addressed messages for it. The Moore system requires the receiver to be on or energized much longer than is actually necessary to receive a message.
The paging system of U.S. Pat. No. 4,437,095 to Akahori et al. is similar in operation to Moore et al. The Akahori et al. page receiver requires its receiving circuitry to power up periodically to detect a synchronization signal and then again a predetermined time later for group messages. With this approach, the Akahori et al. receiver is able to reduce its power consumption only by about half over continuously-powered receivers.
U.S. Pat. No. 4,383,257 to Giallanza et al. discloses a variation of the previous approach. The receiving circuitry in Giallanza et al. is sequentially activated and deactivated in a periodic duty cycle. It is activated in time to detect a synchronization signal that is transmitted by a transmitter whenever a message is to be sent to a receiver. If a synchronization signal is detected, the receiver remains activated beyond its duty cycle to determine if succeeding address signals identify such receiver to continue receiving messages. This approach also requires the receiving circuitry to be on at least a fixed amount of the time regardless of whether messages are being sent to it. In most cases, this on-time is much longer than is necessary to actually receive the transmitted message.
Synchronizing a receiver to a transmitter by use of a real time signal is known in the prior art, but such apparatus requires the receiver to be continuously on. For example, U.S. Pat. No. 4,358,836 to Tohyama et al. discloses an electronic watch that receives a real time signal from a transmitter to synchronize its internal clock. Similarly, U.S. Pat. No. 4,337,463 to Vangen discloses a time synchronization system for synchronizing clocks at remote stations to a clock at a master station.
U.S. Pat. No. 4,419,765 to Wycoff et al. discloses a power-limited paging receiver that also has frequency scanning capability. If an incoming signal is not detected on a present channel, the receiver can scan over several channels. However, the scanning is done blindly. As a result, this scanning consumes power unnecessarily.
Another drawback of the above devices that use dedicated time slots to receive messages is their limited receiving capability. Messages that cannot be transmitted in a single time slot may require several transmission cycles to complete.
U.S. Pat. No. 4,519,068 to Krebs et al. discloses a method for sending messages of a variable length. Krebs et al. transmits data messages which have several fields, including a sync field to synchronize the receiver to the transmitter and data blocks which follow the sync field. The first channel data block includes the station address. The second channel data block includes an information field that indicates the number of following channel data blocks. Krebs et al., however, is impractical for time division multiplexing because of its format.
In known portable paging receivers, which typically operate in the 150, 200 or 400 megahertz frequency bands, the antenna usually comprises a conductor wrapped around a ferrite bar. This antenna, together with the associated paging receiver, is mounted within a nonconducting enclosure sized to fit in a pocket or clip on a belt. Miniaturization of the enclosure beyond this size is limited by the comparatively bulky ferrite antenna that must be contained.
The pager size problem is further aggravated as the paging frequency is lowered. Lower frequency receivers use larger inductors, capacitors and filters in their frequency dependent circuits. Lower frequency paging is desirable, however, because of its superior radio signal propagation characteristics.
In a paging system marketed by the Telecommunications Group of American Diversified Capital Corp., paging data is believed to be encoded on a 57 kilohertz subcarrier in an FM broadcast signal and transmitted at a rate of 1200 baud. Signal modulation is believed effected by phase modulating the 57 kilohertz carrier. If consecutive data elements are identical, the train of 57 kilohertz cycles repeats without interruption. If, however, the data changes state, i.e., from a 0 to a 1 or 1 to a 0, the phase of the 57 kilohertz subcarrier is suddenly reversed. This is believed effected by doubling the length of the positive going or negative going cycle of the subcarrier, thereby introducing a brief DC component into the subcarrier signal. The phase of the subcarrier is thereafter shifted 180.degree. relative to the previous subcarrier phase.
The American Diversified system suffers from a variety of drawbacks. The slow baud rate limits severely the number of users who can be served effectively and the rate at which information can be transmitted. Additionally, it is known that to try to increase the number of users by increasing baud rate and shortening message length reduces reception reliability. This problem is most apparent when using a mobile RF receiver to receive very high frequencies such as FM. Furthermore, the phase modulation technique employed generates broadband spurious components which must be filtered with elaborate filtering circuits so as to reduce interference with the broadcast audio. These filtering circuits increase the cost and complexity of the modulator unit. Extensive filtering must also be provided in the receiver circuit to separate the desired paging information, modulated around 57 kilohertz, from the broadcast stereo audio, which ends at 53 kilohertz. This again increases the costs and complexity of the system.
Accordingly, a need remains for a versatile, wide-area paging system that overcomes the foregoing and other drawbacks of prior paging systems.